A whopping 2.5 billion fully grown T. rexes walked the Earth in the course of the species’ existence, paleontologists found

Jurassic Park T Rex
A T. rex depicted in the 1993 film “Jurassic Park.”

An adult Tyrannosaurus rex required a lot of space – and the prey therein – to survive.

According to new calculations from paleontologists the University of California, Berkeley, each adult T. rex lived in an area roughly 40 square miles in size.

The study, published Thursday in the journal Science, uses that math to offer an estimate of the total number of these predators that walked the Earth during the Cretaceous Period, between 66 and 68 million years ago: an impressive 2.5 billion.

“The total number did catch me off guard,” Charles Marshall, a paleontologist at Berkeley who co-authored the study, told Insider.

Marshall’s analysis suggests that the entire island of Manhattan or city of San Francisco would be the territory of a single T. rex.

He said he’d been wondering for years how unusual it really is to find a T. rex fossil: “When I hold a fossil in my hand, I always said to myself, ‘I know this is freakishly rare.’ But just how rare is it – one in a million or one in a trillion?”

Comparing the T. rex to the Komodo dragon

The T. rex was one of the largest carnivorous land animals that ever walked the Earth. (That accolade currently goes to the polar bear.) An adult Tyrannosaurus rex could weigh at least 5 tons. It stood about 12 to 13 feet tall at the hip and was about 40 to 43 feet long.

The larger predators are, the fewer of them can live in the same area, since there just isn’t enough food to sustain their massive size. This is known as Durham’s Law. So if researchers know how many calories a meat-eater needs to survive, they can calculate the number of predators per square mile.

While there’s no living predator that resembles the T. rex in size, Marshall compared the dinosaur’s energy needs to those of a Komodo dragon, the largest lizard on Earth.

Komodo Dragon
A Komodo dragon in Indonesia’s Komodo National Park.

Using that benchmark, he calculated that there could have been roughly 3,800 T. rex in an area the size of California at any given time – or just two in an area the size of Washington, DC.

There were about 20,000 adult T. rexes living at one time

Marshall’s team also needed to calculate three other variables to determine the total number of rexes that ever walked the planet: the total land area of suitable T. rex habitat, the dino’s average life span as an adult, and how long these predators existed on Earth.

T.rex illustration
A full-grown Tyrannosaurus rex weighed about 6 to 9 tons, stood about 12 to 13 feet high at the hip, and measured about 40 to 43 feet long.

By reviewing the locations of every T. rex fossil ever found, the study authors determined that the predator lived in about 888,000 square miles of North America – and nowhere else on Earth. Although the animal might have been able to survive in the area that’s now Siberia, Marshall said, he’d be surprised if any rex fossils were ever found outside the one continent.

Using that assumption about the T. rex’s geographic range, Marshall calculated that there could have been about 20,000 adult T. rexes alive at any given time in the species’ existence.

Figuring out how long the T. rex species was around was a bit easier – the oldest rex fossil ever found suggests the dinosaur walked the Earth for the last 2.5 million years of the Cretaceous Period, starting 68 million years ago. Then it went extinct after the Chicxulub space rock struck.

Finally, Marshall’s team calculated how long one generation of adult rexes lasted by looking at the average time span between when rexes became fully grown – at around age 15 – and when they died in their early 30s.

woodward4HR
An artist’s depiction of a juvenile Tyrannosaurus rex.

That means that in the 2.5 million years these animals were on Earth, there were about 127,000 generations of them. Multiply that number by 20,000, and you wind up with 2.5 billion T. rexes.

However, Marshall pointed out that this number doesn’t include baby or juvenile T. rexes. Those were excluded from the calculations because research suggests juvenile rexes were smaller and faster than their adult counterparts, so hunted different prey.

T Rex in New York City
A Tyrannosaurus rex skeleton, named STAN, displayed by Christie’s Auction House in New York City, September 15, 2020.

Marshall’s team’s estimate suggests that the remains of just one in every 80 million adult T. rexes have been found. Currently, there are about 32 well-preserved, adult T. rex skeletons in public museums worldwide, Marshall said.

That means we’ve only dug up 0.00000125% of all the adult T. rexes that ever lived.

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The space rock that doomed the dinosaurs was shrapnel from a comet that flew too close to the sun, a Harvard study suggests

dinosaur asteroid meteor
An artist’s depiction of the moment the Chicxulub asteroid struck present-day Mexico 66 million years ago.

About 66 million years ago, a space rock more than 6 miles wide collided with Earth, striking land that is now part of Mexico.

The impact sparked wildfires that stretched for hundreds of miles, triggered a mile-high tsunami, and released billions of tons of sulfur into the atmosphere. That gaseous haze blocked the sun, cooling the Earth and dooming the dinosaurs, along with 75% of all life on the planet.

But the origins of that dinosaur-killing rock, named Chicxulub, have remained a mystery. 

Most theories suggest Chicxulub was a massive asteroid; hundreds of thousands of these rocks sit in a donut-shaped ring between Mars and Jupiter. But in a study published Monday, two Harvard astrophysicists suggested an alternate idea: that Chicxulub wasn’t an asteroid at all, but a piece of shrapnel from an icy comet that had been pushed too close to the sun by Jupiter’s gravity.

Asteroids and comets are both classified as space rocks by NASA, but they differ in key ways: Comets form from ice and dust outside our solar system and are generally small and fast-moving, whereas rocky asteroids are larger, slower, and form closer to the sun.

“We are suggesting that, in fact, if you break up an object as it comes close to the sun, it could give rise to the appropriate event rate and also the kind of impact that killed the dinosaurs,” Avi Loeb, an astrophysicist and cosmologist at Harvard University and co-author of the new study, said in a press release

The solar system acts like a ‘pinball machine’ for comets

asteroid meteor armageddon shutterstock
An artist’s depiction of an asteroid approaching Earth.

Most asteroids come from the asteroid belt between the solar system’s inner and outer planets. But NASA scientists who keep tabs on space objects that pass near Earth have yet to figure out where Chicxulub came from. 

In the new study, published in the journal Scientific Reports, Loeb and his co-author, Amir Siraj, suggest Chicxulub didn’t come from the asteroid belt. Rather, they say it more likely originated outside our solar system, in an area called the Oort cloud

Think of the Oort cloud as ring made of 1 trillion pieces of icy debris, which sits beyond the farthest reaches of the solar system, surrounding it. It’s located at least 2,000 times farther away from the sun than Earth is. Comets that originate in the Oort cloud are known as long-period comets because they take so long to complete one orbit around the sun.

But these comets can sometimes get pulled off-course by the gravity of massive planets like Jupiter. Such a tweak to a comet’s orbit could send it hurtling on a path much closer to the sun. 

“The solar system acts as a kind of pinball machine,” Siraj said in the release.

Comets that get near the sun are called “sungrazers.” The new study calculated that about 20% of Oort cloud comets are sungrazers. As they approach our star, its gravity starts to pull them apart. Fragments of comet slough off and may careen toward nearby planets. 

This, the study authors say, is “a satisfactory explanation for the origin of the impactor” that killed the dinosaurs.

The asteroid-versus-comet argument isn’t settled

Chicxulub_impact asteroid
A painting depicting an asteroid slamming into tropical, shallow seas of the Yucatan Peninsula in what is today southeast Mexico. The aftermath is believed to have caused the extinction of the dinosaurs.

Siraj and Loeb aren’t the only scientists who think a comet, not an asteroid, doomed the dinosaurs. A group of researchers from Dartmouth College similarly suggested in 2013 that a high-speed comet could have created the Chicxulub crater. 

Chicxulub hit Earth at a speed of 12 miles per second (43,200 mph), which is about 30 times faster than the speed of a supersonic jet. The resulting 100-mile-wide crater extended 12 miles into the depths of the Gulf of Mexico. Some scientists have estimated the asteroid’s power was equivalent to 10 billion of the atomic bombs used in World War II.

But not all researchers are convinced a comet caused that destruction.

Natalia Artemieva, a senior scientist at the Planetary Science Institute in Arizona, told The New York Times that comet fragments from a sungrazer would have been too small to create the Chicxulub crater. And Bill Bottke, a planetary scientist at the Southwest Research Institute in Colorado, suggested that the study overestimates the frequency of sungrazers – and, consequently, the amount of fragments those comets produce.

Existing evidence favors the idea that Chicxulub was an asteroid, “but it’s not conclusive,” Bottke told the Times. “There’s still wiggle room if somebody really wants it to be a comet. I just think making that case is really hard.”

Siraj and Loeb, however, said their theory is supported by a type of material found deep inside the Chicxulub crater and other craters in South Africa and Kazakhstan. That substance, carbonaceous chondrite, may have come from comets. Whereas just 10% of asteroids from the asteroid belt are composed of carbonaceous chondrites, the material “could potentially be widespread in comets,” the study authors wrote.

The only samples ever collected from a comet in space were brought back in 2006. They revealed that object, called Wild 2, was composed of carbonaceous chondrite.

kuiper belt oort cloud
Artwork depicting the icy cores of baby comets beyond Neptune at the edge of our solar system.

Finding the correct answer in the Chicxulub debate is useful because it could help researchers figure out the likelihood of a similar impact event in the future. Only two to three comets from the Oort Cloud have hit Earth during the last 500 million years, according to one study. By contrast, according to the Planetary Society, a Chicxulub-sized asteroid impacts Earth every 100 million years or so

Siraj and Loeb modeled how many long-period comets get close enough to the sun to shed large fragments in the direction of Earth. Their numbers suggest 10 times more Chicxulub-sized objects hit Earth over its history than scientists previously thought.

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